Method of electroless deposition of metals with improved sensitizer

ABSTRACT

METHOD FOR AUTOCATALYTICALLY ELECTROLESSLY PLATING A DIELECTRIC SURFACE WITH A METAL SUCH AS NICKEL, COBALT OR COPPER COMPRISING SENSITIZING THE SURFACE WITH AN IMPORVED SENSITIZING SOLUTION COMPRISING BOTH DIVALENT TIN ION AND TETRAVALENT TIN ION, THE SOLUTION HAVING A PH LESS THAN ABOUT 1.5, TREATING THE SENSITIZED SURFACE WITH A CATALYZING SOLUTION TO PROVIDE CATALYTIC NUCLEATING CENTERS, AND PLATING THE METAL ON THE CATALYZED SURFACE. THE IMPROVED SENSITIZING SOLUTION PERMITS PLATING ON SMOOTH, DENSE, WATER REPELLENT PLASTIC SURFACES SUCH AS TEFLON, LUCITE, SILICONE RUBBERS AND MYLAR.

United States Patent METHOD OF ELECTROLESS DEPOSITION OF METALS WITHIMPROVED SENSITIZER Nathan Feldstein, Kendall Park, and Thomas StephenLancsek, Mercer, N.J., assignors to RCA Corporation No Drawing. FiledJuly 31, 1970, Ser. No. 60,091

Int. Cl. B44d 1/092; C23c 17/02 US. Cl. 117-47 A 8 Claims ABSTRACT OFTHE DISCLOSURE Method for autocatalytically electrolessly plating adielectric surface with a metal such as nickel, cobalt or coppercomprising sensitizing the surface with an improved sensitizing solutioncomprising both divalent tin ion and tetravalent tin ion, the solutionhaving a pH less than about 1.5, treating the sensitized surface with acatalyzing solution to provide catalytic nucleating centers, and platingthe metal on the catalyzed surface. The improved sensitizing solutionpermits plating on smooth, dense, water repellent plastic surfaces suchas Teflon, Lucite, silicone rubbers and Mylar.

BACKGROUND OF THE INVENTION Certain metals can be plated on non-metallicsurfaces by an autocatalytic electroless process in which the surface isfirst treated with a sensitizing solution, then with an activatingsolution which precipitates nucleating centers. In this way ceramics andmany synthetic resins can be provided with coatings of nickel or copper,for example, which may or may not be later covered with a heavier layerof metal by electroplating.

The sensitizing solution most commonly used in the past contains Sn+ions generally formed from the salt stannous chloride. This material hasproved to be quite satisfactory for electroless plating of metals onmost nonmet-allic materials. However, there are certain plastics, suchas Teflon, Lucite and silicone rubbers, as well as certain commercialphotoresists, which cannot be satisfactorily plated by this method. Thereason for the difficulty appears to be that these materials areunusually smooth and non-porous and are not readily wetted by aqueoussolutions of stannous salts. As a result, when one attempts to depositmetal autocatalytically on one of these materials with no surfacetoughening, using conventional stannous salt sensitizers andconventional activators, such as palladium chloride, plating is spottyand otherwise non-uniform.

It should be noted that, in general, those who have used acidicsolutions of stannous salts as electroless plating sensitizers, havetried to keep the tin ion in the divalent state by methods such askeeping metallic tin or other reducing agents such as sugars in theshelf stock solutions.

OBJECTS OF THE JNVENTION One object of the present invention is toprovide an improved method of autocatalytically electrolessly depositingmetals on certain non-metallic substrates which are difiicult to wet.

Another object of the invention is to provide an improved sensitizingsolution for autocatalytically electrolessly depositing metals onnon-metallic substrates.

DESCRIPTION OF PREFERRED EMBODIMENTS It has now been found,unexpectedly, that if stannic ion is added to conventional acidsensitizing solutions containing stannous ion, and the resultingsolution is used to treat difiicult-to-plate non-metallic surfaces, inthe usual way, these sensitized surfaces (after the usual activation3,666,527 Patented May 30, 1972 "ice Example I Substrate: smooth Teflonplate with no mechanical or chemical roughening.

Procedure: after the Teflon plate was chemically cleaned with adetergent solution it was immersed in a sensitizing solution comprisingSnCl having the concentration of 6.4)(10 moles/liter, an HCl content of0.19 mole, and varying amounts of SnCl The treated plate was removedfrom the sensitizing bath, rinsed and immersed in an activating bath ofPdCl containing 1 g. of PdC12 and 1 cc. conc. HCl per liter (remainderwater). .After a few seconds in the activating bath, the plate was againremoved and rinsed, after which it was immersed in a nickel-depositingbath at 25 C. having the following composition:

In one liter of solution:

Dimethylamine borane1.5 g. NiSO .6H O-25 g. Na 'P O .10I-I O-50 g. NH OH(58% conc.)25 cc. Water-remainder.

The plate is permitted to remain in the plating bath until either a gooduniform plating is obtained or it is apparent that no further deposit isbeing obtained on unplated areas. This is usually a few minutes.

The following table shows the effect of adding varying amounts of SnCL,solution to a conventional SnCl sensitizing solution. In this case, theSnCl, stock solution was prepared by dissolving crystalline material inwater and permitting the solution to stand for about one week at roomtemperature before using. It has been found that an aging effect, notfully understood, occurs in this solution. Less of the aged solution isrequired to impart the improved wetting to the sensitizer. The agingefiect may be due to an hydrolysis reaction taking place.

Cone. of SnCL, (moles/liter): 1 I Plating results 0 Practically nowetting or plating. 2.5 X 10* Spotty plating. 5.0x l0- Do. 7.5 x 10 Do.1.25 X 10* Do. 2.0)(10' Good plating. 2.5 X 10- Do. 325x16 Do. 3.75 10Do.

In the sensitizing bath which also contained 6.4)(10- moles/liter ofSnClz.

NOTE.AS the amount of SnCli was increased, coverage of plating becamebetter ni gradual increments. There was actually no abrupt change fromspotty to good."

Example II With all conditions and treating and plating solutionsremaining the same as in Example I except the proportion of stannic ion,Mylar tape coated with AZ-l350, a positive acting photoresist, wasplated with nickel from the same nickel-boron bath. In this case, whenthe concentration of stannic ion was above zero and up to '7.5X10-moles/liter, only patchy plating was obtained. Above about 1.25 Xmoles/liter good plating was obtained although this increase in coveragewas again gradual. With no stannic ion added, only little plating wasobtained.

Example III In another test run using a nickel-boron plating bath, asdescribed in Example I, on AZ-1350 photoresist, the sensitizing solutionwas made up by adding varying amounts of powdered SnCl -5-H O (reagentgrade) to freshly prepared SnCl solution. The stock solution from whichthe stannous ion solution was prepared comprised 214 gms. SnCl -2H O(Baker reagent grade) and 290 cc. conc. HCl. The actual solution usedfor the run contained 64x10 molar concentration of Sn+ ion and an HClcontent of 0.19 M.

Patchy plating was obtained with molar concentrations of Sn ion belowabout 0.86X 10- Above about 0. 86Xl0- molar, satisfactory, uniformplating resulted.

Activation procedure was the same as in Example 1.

Example IV iWith conditions otherwise the same as in Example HI, themolar concentrations of Sn+ ion used was increased to 12.8Xl0- molar andthe HCl to 0.38 Patchy plating on AZ-1350 photoresist from the samenickelboron plating bath was obtained using molar concentration of Sn+ion below about 10.4)(10' molar. Good, uniform plating was obtainedabove about 104x10 molar.

Example V Copper was deposited on a smooth Teflon substrate using thefollowing procedure. After cleaning the substrate, it was immersed in asensitizer solution containing Sn and Sn+ ions as set forth below,treated with a PdCl activating solution as in the previous examples, andthen immersed in a copper plating bath. The copper plating bath had thefollowing composition.

Per liter CuSO '5H- O -.gms-.. 7.5 Ethylene diaminetetra-acetic acid(E.D.T.A.) (40% solution of the sodium salt) -gm-s 36.5

NaOI-I g s 20.0

NaCN gm 0.1

HCHO (37% solution) ml 40 Water to make 1000 ml.

The sensitizing solution contained Sn+ ion 6.4X10- molar and had an HClcontent of 0.19 molar. To this was added varying amounts of stannicchloride solution that had been aged for a minimum of one week.

(Gradual increase in plating coverage with increasing amounts of Sn+ion.)

Example VI Using the same copper-depositing bath as in Example V and Sn+ion concentration of 6.4X10- moles, an experimental run was made to seeif the quality of the plating was affected by using increasingly highratios of Sn ion to Sn+ ion, with the following results. The substrateused was Teflon.

.4 Cone. of Sn+ ion (molar) 2 Plating results 8.0 X 10- Good. 1 X 10-Do. 2 X 10 Do. 3 X 10- Do. 4x10 Do. 5 X 10- D0. 6 X 10- iBorderline. 7 X10-- Patchy. 8 X 10- Do.

It was therefore concluded that, in each case, there is likely to be anupper limit to the amount of Sn+ ion in relation to Sn+ ion but thiswill differ with different plating solutions, different substrates andother conditions.

Although a preferred way to add the Sn* ion is by a solution which hasbeen aged, since less tin (-l-IV) is required using this procedure,there are other ways in which stannic ion may be generated. One of theseis by adding agents capable of oxidizing the Sn ion to Sn ion, such asiodine or permangauates, for example. Another method is by exposing theSn+ solution to U.V. light for a period of time. By merely heating theSn+ solution in air, some of the Sn ion is converted to Su In practice,the addition of Sn ion is preferred because the amount can be moreaccurately controlled and can conveniently be used in productionfacilities.

For further information on other previously-known stannous chloridesensitizing solutions to which stannic ion can be added as describedherein, see, for example, Metallic Coating of Plastics by WilliamGoldie, Electrochemical Publications Limited, London (1968) Chapter 5.

In the examples given above, the pH of the solutions was generally belowone and the solutions were clear. At pH values around 1.5 the solutionsbecome turbid. It is therefore preferred that the pH value he kept belowabout 1.5 and, more preferably, below one.

In the present method it is believed that the improved results are dueto the presence of a complex of Sn+ and Sn.

In general, a molar ratio of divalent tin ion to tetravalent tin ionbetween about 1:1 and 1000:1 has been found to be operative.

Although stanic chloride is given as an example of a suitable tincompound to use to add So ion in the present invention, it will beunderstood that other soluble stannic compounds can be used.

The improved method of the present invention is also of particularinterest in the making of printed circuit boards; especially boardshaving small-diameter through holes which must be plated with metal.Materials commonly used for printed circuit substrates are phenolicresinimpregnated fibrous materials, epoxy resin-impregnated boards andglass fiber-epoxy resin boards. The boards are usually laminates ofseveral thin sheets. The improved sensitizers described herein wet thesematerials very well. The sensitizers wet so efliciently that very smalldiameter through holes, usually very diflicult to plate electrolesslywith previously used Sn+ sensitizers, can now be plated very well. Theimprovement is due not only to the increased wetting ability of thepresent sensitizers but also due to the fact that they are not degradedby air agitation. Agitation of the bath is necessary to cause solutionsto flow through the holes. Air agitation adversely affects Sn+sensitizers.

What is claimed is:

1. A method for electrolessly plating a surface with a metal comprisingthe steps of sensitizing the surface with a sensitizing solution made bymixing separate sources of divalent tin ion and tetravalent tin ion in amolar ratio from about 1:1 to about 1000: 1, the solution having a pHlower than about 1.5, the tetravalent tin ion being prepared as aseparate solution of a stannic salt which is aged for a time beforecombining it with the source of divalent tin ion, treating thesensitized surface with a catalyzing solution so as to provide catalyticnucleating centers thereon, and plating said metal on the catalyzingsurface by contacting the surface with an electroless metal platingbath. 2. A method according to claim 1 in which said surface is asmooth, dense, difficultly wettable plastic.

3. A method according to claim 2 in which said surface is Teflon.

4. A method according to claim 1 in which said metal is nickel.

5. A method according to claim 1 in which said metal is copper.

6. A method according to claim 1 in which said surface is a phenolicresin impregnated fibrous material.

7. A method according to claim 1 in which said surface is an epoxyresin-impregnated material.

8. A method according to claim 1 in which said surface is a glassfiber-epoxy board.

References Cited UNITED STATES PATENTS 2,282,511 5/1942 Bradley 1171303,033,703 5/1962 Schneble et al 117-47 3,134,690 5/ 1964 Eriksson 117-47X 3,167,491 1/1965 Harrison et al. 117138.8 X 3,245,826 4/1966 Luce eta1 117-47 3,573,973 4/1971 Drotar et al. 117-47 A 3,616,296 10/1971Bernhardt et al. 117-47 A T. G. DAVIS, Assistant Examiner US. Cl. X.R.

117138.8 R, 138.8 UA, 138.8 UP, 160 R, 213

